As crazy as this may sound, they may not see “mono” as a big selling point anymore in 2017. It should obviously be included, but they are probably afraid uninformed people will listen to the mono track by accident and hear how flat it sounds quality wise (even if the sound effects are Superior) and be turned off. I’m sure someone out there has made a patched version of the 5.1 mix and fixed the effects though.
I understand that it’s not a selling point to anyone except purists, but there’s no reason to leave it off the disc.